A suspension of a cis[((1R,2R)-1,2-cyclohexanediamine-N,N′)bis(R1)]platinum(II), which is a platinum complex, in an iodine addition product of the ethyl ester of the fatty acids of poppyseed oil is known as a pharmaceutical agent showing a superior effect to conventional pharmaceutical agents in liver cancer, malignant lymphoma, non-small cell lung cancer, small cell lung cancer or superficial bladder cancer. Particularly, a suspension of a cis[((1R,2R)-1,2-cyclohexanediamine-N,N′)bis(tetradecanoyloxy)]platinum(II) having the following structure in an iodine addition product of the ethyl ester of the fatty acids of poppyseed oil is known to show a superior effect:

A suspension of a cis[((1R,2R)-1,2-cyclohexanediamine-N,N′)bis(R1)]platinum(II) in an iodine addition product of the ethyl ester of the fatty acids of poppyseed oil is administered as an injection. Accordingly, a step for sterilization is necessary. As a method for sterilization, a method comprising dissolving in a solvent, followed by filtration is typical. In the case of cis[((1R,2R)-1,2-cyclohexanediamine-N,N′)bis(R1)]platinum(II), for example, 2-methyl-2-propanol, chloroform and the like are solvents for dissolution. As a preferable solvent, 2-methyl-2-propanol is described in JP-B-2886247. However, when 2-methyl-2-propanol is used for sterile filtration and a lyophilized formulation for injection is prepared, and then the formulation is suspended in an iodine addition product of the ethyl ester of the fatty acids of poppyseed oil, problems occur in that the viscosity increases with time and the suspension separates into two layers when left standing for a long time.